The issue regarding mobile phones I believe is the small possibility of dropping the phone, having the battery become detached from the phone and the terminals on the battery shorting on something.

With the way some people drive when they are talking on their cellphone, maybe this is a practical consideration. You wouldn't want someone on their cellphone to pull out the pump nozzle while it is still pumping gas, or to simply forget that the nozzle was still in the tank and drive off.